Afpectus Lunae: Does the Moon Rotate on Its Axis?

Last summer I often walked down the rough path to our pond and stepped out onto the dock. There I would lie down on my back and stare up at the night sky. The low rumble of the distant highway was easy to shut out when a shooting star flew by. The bright face of the moon often sparked my curiosity. Looking at it made me think about the telescope that sat in our closet, gathering dust. A few years ago my grandfather gave my family a nice backyard telescope for Christmas. Unfortunately, it sat in the house for four years because it was "too much work to figure out." This fall I set it up to do a school project with my science teacher. I wanted my project to be centered on the moon. Obvious possible topics were the moon's phases, the rising and setting of the moon, and lunar and solar eclipse phenomena. But after more pondering, I realized that I did not know if the moon, like Earth, rotates on its axis. I thought that I might be able to discover this simply by viewing the moon over several months and recording its appearance. My prediction was that if the moon rotates, the appearance of its surface would change, and if it does not rotate, the surface of the moon would always look the same. I hypothesized that the moon rotates because the same forces that affect Earth should affect the moon.

An astrolabe is a homemade tool used to record the moon's elevation in the sky. (Click to enlarge)

I planned to observe and record the appearance of the moon's surface as often as weather conditions and time permitted during October, November, and December 2003. The kit I created for viewing the moon included a compass, a homemade astrolabe, two different sizes of telescope lenses (9mm and 25mm), several pencils, and my field journal, which consisted of data sheets. At the top of each of these sheets I left a space to sketch the moon. From these sketches I hoped to confirm my hypothesis.

Most "viewing nights" began with my checking a moon phase calendar to see where in the sky the moon would be and in what phase it would be. Then I took our telescope out onto our lane. We live in the country about six miles from a small town, so there is no significant light to obscure my view. The only obstructions are all the trees, so the cleared lane was an ideal viewing spot.

After leveling the telescope, I got out my tools. Using the lower-magnification lens (9mm), I located the moon and focused the telescope. Then, in order to draw the details of the moon accurately, I replaced that lens with the higher-magnification lens (25mm) and refocused the telescope.

Sample Data Sheet (Click to enlarge)

On my data sheets, I recorded the date, time, weather, location of viewing, compass direction, and the elevation of the moon above the horizon. At first I used the altitude scale on my telescope to record the elevation of the moon. Later, my teacher showed me a science activity book that led me through the steps of making an astrolabe. While the astrolabe functioned no better than the telescope, it was more fun to create my own simple tool.

The next spot on my data sheet was for recording my observations. I tried observing the moon with my unaided eye, but I needed more detail for my drawings. Using binoculars was cumbersome because I could not observe and draw at the same time. The telescope freed my hands for drawing and gave me the necessary magnification; therefore, I used it exclusively for the remainder of my observations. The last step for filling out a data sheet was to sketch the moon. I made clear, simple drawings with a focus on distinct surface features.

During this process I became familiar with the main features of the moon and their appearance. Whenever I saw a picture of the moon in a book or some other place, I was able to recognize some of the features and noticed that all the sources depicted the same view of the moon.

Sketches from Olivia's field journal.

The simplest feature for me to locate is called the Sea of Crises. The features on the moon were named hundreds of years ago when they were thought to be seas. In my observations, this particular "sea" was always in the top left-hand portion of the moon. Being able to recognize certain features gave me a great sense of satisfaction. Often I ran excitedly to my mom, dad, or best friend, pointed at the sky and said, "Ah! Look there. That's the Sea of Serenity! Isn't that cool!" They would look back at me blankly and shake their heads, but I think they understood. After viewing the moon for a few nights and noting its depiction in other sources, I began to think that my hypothesis was false. If the moon rotates on its axis, I should have seen different features of the moon on each viewing. So far, the nearside of the moon had not changed at all. ("Nearside" refers to the portion of the moon exposed to Earth. I will use this term henceforth.)

Did the moon spin on its axis, or not? It sure didn't appear to, but it seemed odd that the moon would hang still in space. I continued to think that since the Earth spins on its axis and the moon is affected by the same forces as the Earth, the moon should spin as well. Like any good naturalist, I went to the "literature." The first place I looked was in a children's book entitled The Moon Seems to Change. This gave me a quick and easy overview of the moon. I learned that the moon's "day" is almost one month long. If the moon could have a "day," it must rotate on its axis! I was thrilled to discover that the moon did rotate. Now that that question was answered, I realized that I would have to continue to investigate with a new question in mind: Why does the moon rotate on its axis, and yet observers on Earth only see one view? With more reading I learned that it takes about the same amount of time for the moon to rotate on its axis (27.3 days) as it does for the moon to make one revolution around Earth (29.5 days). I wondered if this was just a coincidence, or if this piece of information was going to prove to be valuable. One thing that struck me about this information was that the moon is, in this respect, very different from Earth. Earth takes dramatically different amounts of time to rotate on its axis (24 hours) and to revolve around the sun (364.25 days).

Olivia views the moon through a telescope.

Since I didn't learn the answer to my new question from this reading, I decided to search the Internet for information. I learned that the moon rotates but does it at such a slow speed, relative to Earth's speed, that it always keeps the same part facing Earth. I had trouble picturing this, so I decided to prove it to myself by making a model. I took two oranges and conducted an experiment. First I put a dot on one side of one orange. That orange was Model Moon, and the dot represented the nearside (visible surface) of the moon. I then set the other orange, Model Earth, on the table and made a circular path around it with Model Moon, always keeping the dot facing one direction. In other words, the moon was not rotating. Model Earth was exposed to different views of Model Moon. But when I revolved Model Moon around Model Earth and at the same time slowly rotated it, the dot always faced Model Earth. It made so much sense now! The moon does rotate on its axis. One rotation takes nearly as much time as one revolution around Earth. If the moon were to rotate quickly (several times each month) or not rotate at all, Earth would be exposed to all sides of the moon (i.e. multiple different views).

The Model Moon experiment.

With further research I learned that millions of years ago the moon actually did rotate much faster relative to its current speed. Over time it has slowed down because of the effect of Earth's gravity. Astronomers call this a "tidally locked" state because it will now remain at this speed.

Looking back at my hypothesis, I found that it was correct, but my predictions as to how I would discover this were not. I've been told that astral observations often lead to counterintuitive conclusions, and my experiment was no exception. Originally, I was unable to see what was ultimately made obvious by my research and my model using oranges.

Several days after drawing these conclusions, I interviewed Dr. Norman Siems, a professor of astronomy at Juniata College in Huntingdon, PA. I wanted to confirm that all my conclusions were correct and that there was nothing left unfinished. Professor Siems and I discussed all my findings up to this point, and he confirmed my conclusions. Then he brought up a very important point. How could I be sure that the moon didn't simply make one full rotation a day? Maybe that was why I saw the same features with each viewing. For a moment I was baffled. Then it came to me that 24-hour rotations would only be possible if I viewed the moon every night at the same time and saw the same features. If it truly did take the moon 24 hours to make one rotation on its axis, the moon would look different when I viewed it at 8 p.m. compared to when I viewed it at 11 p.m. And it didn't. The moon always looked the same, regardless of the time or phase.

The Model Moon experiment.

After ruling this possibility out, Dr. Siems and I considered whether there were any other situations that would fit the data I had collected. We could not think of any other possibilities, so we resolved that I had narrowed everything down to my answer. The moon rotates, but in doing so always keeps the same face toward Earth. Now sometimes I walk down to our pond and lie back on the dock. I look up at the sky. A shooting star flies by, and I turn my attention to afpectus lunae, the face of the moon. I have learned much in the past few months about the moon, from its countless myths to its countless facts. But knowledge about the moon has not been all that I have gathered. I have also learned how to use a telescope and have gotten hands-on experience with the scientific method. I have been introduced to a new field, astrophysics. And best of all, I have had an unbeatable excuse to stay up late. "Oh, too bad, it'll be a while before I can go to bed, Mama. I have to view the moon."

More About This Resource...

This winning entry in the Museum's Young Naturalist Awards 2004 investigates why we see only one view of the Moon's face. Olivia's narrative essay (with photographs, illustrations, and references) discusses:

her realization that she didn't know whether the Moon, like Earth, rotates on its axis

her prediction that she could determine if the Moon was rotating by looking for changes in the appearance of its surface

the two-month observation she planned and the kit she created for viewing the Moon

the early results of her experiment and the research discovery that led her to change her question to: Why does the Moon rotate on its axis, and yet observers on Earth only see one view?

Completion Time

Less than 1 period

Teacher Tip

Supplement a study of astronomy with an activity drawn from this winning student essay.

Tell or remind students that the Moon rotates on its axis. Then ask: So why do observers on Earth only see one view?

Send students to this online article, or print copies of the essay for them to read.

Divide the class into small groups, and have students re-create the model from the article.